This invention relates to a highly active palladium catalyst, in which spinels are used as carrier material, for the preparation of cyclohexanone by selective vapour phase hydrogenation of phenol and to the process for using the said catalyst.
In U.S. Pat. No. 3.305.586 it has been disclosed that phenol can be hydrogenated to cyclohexanone by means of a catalyst of palladium on alumina as carrier. The process provides either high conversion rates with low selectivity or high selectivity with low conversion rates. The degree of purity in which the cyclohexanone is obtained from the hydrogenation process is of great economic importance for the working up of the hydrogenation product in order to obtain pure cyclohexanone.
According to U.S. Pat. No. 3.076.810 which describes a process for the preparation of cyclohexanone from phenol, an improvement in yield is obtained if the alumina used as carrier material is impregnated with 0.1 to 1% by weight of sodium in the form of sodium carbonate or sodium hydroxide. In order to obtain sufficiently high selectivity at these high conversion rates, however, the hydrogenation of phenol over this catalyst requires residence times of from 21/2 to 5 hours, which are too long to be of technical interest. British Patent Specification No. 1.063.357 describes a process for the preparation of cyclohexanone by vapour phase hydrogenation of phenol in which the catalyst carrier used is a mixture of 40 to 98% by weight of .gamma.-alumina and 2 to 60 % by weight of alkaline earth metal hydroxide. However, the conversion rates of phenol are low when hydrogenation of phenol is carried out using this palladium-containing carrier.
According to DDR Patent Specification No. 69 585, higher conversion rates and selectivity are achieved if the alumina (40 to 98 % by weight) is mixed with a mixture of alkaline earth metal hydroxides (2 to 60 % by weight), the alumina used containing either .alpha.-alumina or .chi.-alumina addition to .gamma.-alumina.
The catalyst carrier is itself prepared by adding graphite to a mixture of alumina and alkaline earth metal hydroxides and pressing the resulting mixture to form cylindrical carriers. A serious disadvantage of catalyst carriers prepared in this way is that their mechanical hardness is generally low.